Until now, no one had known. But here before me, on scarcely twenty acres of lofty wooded slope in central Mexico, the monarchs crowded by the millions to while away midwinter months in semidormancy.

I am a Canadian zoologist, Toronto-based. With the tireless help of my wife, Norah, I have spent much of my time since 1937 studying the ecology, and especially the migration, of the monarch butterfly.

Monarch migration is a marvelously intricate pattern of behavior, baffling in many of its aspects. The butterfly has long been known to travel great distances, somewhat as birds do, on a round trip keyed to seasonal changes and the reproductive cycle. For the monarch, as for the feathered flocks, southward migration's clear and evident purpose was to escape the killing frosts of winter.

Some monarchs flying south in the fall return to their summer breeding grounds, we knew, though none ever survive longer than a year. Where, then, did the eastern butterflies pass that single overwintering of their brief lives? One of the earliest questions asked, it was to be among the last answered.

Our first problem was to track the insects on their journeys, and plot the distances and directions of their flight; to do that, we had to mark them. But how do you mark a migrating butterfly, a delicate, featherweight insect that depends totally on freedom of flight?

It took many years—and many failures—to develop a foolproof way to tag a monarch. As long ago as 1937 we experimented with a printed label, affixed to the butterfly's wing with liquid glue. But tags and butterflies got tangled and sticky, and many of the insects couldn’t stay airborne.

Norah and I next had labels printed on gummed stock, like postage stamps. We tested them on the Monterey Peninsula in California, where western monarchs from the intermontane valleys of the Great Basin have always congregated in midwinter. This experiment, too, failed dismally: A night of rain washed the gummed labels off the clustered monarchs. Over the sodden grass our thousand tags lay like soaked confetti.

But then a friend suggested trying the type of pressure adhesive label used for price tags on glass merchandise. With an added fixative, this worked perfectly. Now we had a tag that would readily adhere, with a gentle squeeze, to the membrane of a butterfly’s wing where the scales had been removed. The harmless labels even stuck to monarch wings deliberately soaked in water.